With the development of information technology, flat panel display has gradually replaced the conventional cathode ray tubes (CRT) display. Flat panel LCD, for instance, which takes up the largest market share among all flat panel displays, is composed of backlight source, light polarizer, glass substrate, liquid crystal, thin film transistor (TFT), color filter (CF), etc., while the color filter is the key component determining the color characteristics and contrast of a LCD.
Color filters in LCD and coloring unit for the fluorescent film in plasma display panel modules are the key components of the structures that convert black and white flat panel display into colorful ones. The coating structures of color filter for flat panel LCD, for instance, comprises a plurality of pixels of red (R), green (G) and blue (B) colors which are arranged in arrays on glass substrate, while a couple of pixels (normally three) correspond to one color dot on the display. When white light passes through the trichromatic pixels, it generates three primary colors of light, namely the red, green and blue light, which, by means of gray scale effect generated by the liquid crystal molecules, are further blended and form various colors.
The technologies for the fabrication of color filters can be classified into three types. The first coating type is photolithography method, which is the most frequently used technology currently. In the technology, uniform liquid films are coated to the substrate and defined patterns by photolithography method sequentially. This technology is applied to many methods including dyeing method, pigment dispersing method, electro-deposition, etc. Another type of technology is stamping, in which the patterns are respectively decided by stamps and impressed onto the substrate. The third type of technology is ink injection, in which miniscule droplets of ink are injected onto a substrate by ink injecting heads, allowing direct formation of micro patch patterns.
Referring to photolithography technology mentioned above, the prerequisite is to coat a liquid film uniformly. Currently, the most frequently used coating method is spin coating (as disclosed in U.S. Pat. No. 4,451,507). However, due to low material utility rate, the method has recently been phased out by other developments, such as extrusion spin coating (as disclosed in U.S. Pat. No. 6,191,053) and slot patch coating (as disclosed in U.S. Pat. No. 4,938,994). Both inventions aim to improve the material utility rate to allow the formation of uniform liquid film. The difference among the various methods, the dyeing method, pigment dispersing method and electro-deposition, lies in that the coating liquid film materials have different characteristics and accordingly specific operation procedures are applied.
The conventional dyeing method (as disclosed in U.S. Pat. No. 4,744,635) processes a dye absorbing layer made from transparent organic photosensitive material by photolithography and etching to form a pattern. The dye absorbing layer is immersed in a dyeing solution. Then, the display is exposed, dyed, baked and resist dyed to finish. The operation procedures are repeated for three cycles to obtain of three layers of color pattern, the red, green and blue colors. The method is not only too complicated, but also demands the installation of expensive equipment. Besides, because of the poor resistance of dyes against heat and light, the dyeing method is limited to apply for fabrication of small sized colorful LCD and conventional CRT.
Conventional pigment dispersing method (as disclosed in U.S. Pat. Nos. 5,085,973 and 4,786,148) is the most popular method used in manufacturing color filters currently. Photosensitive and thermosetting pigments are used. The procedures comprise coating coloring material to the mask on the glass substrate, and exposure imaging, baking, etc. to produce monochromatic micro-imaged color patch. Three cycles of operation procedures are required to produce trichromatic RGB pixels. The pigment dispersing method is complicated and requires expensive equipment and the operation is time-consuming, and it has low utility rate of coloring material and limited variation in pixel pattern, and therefore this method is not potential to meet the future demands for larger size and lower price display panel.
Known electro-deposition (as disclosed in U.S. Pat. No. 4,522,691) includes generating a patterned and transparent conductive film on a glass substrate and coating the coloring materials thereon by electrophoresis. Similarly, three cycles of the operation procedures are required to produce the patterns in RGB colors. The method also includes photolithography process. Hence, a number of operation parameters are involved, making it difficult to control the yield rate accurately. The inclusion of an additional transparent conductive film set forth by this method is the most significant drawback, as it lowers the light permeability and resolution, and hence it limits the layout of the patterns which cannot be too elaborate.
To conclude, the conventional coating technology fails to define patterns directly at coating, and it relies, instead, on exposure to remove excessive materials. Thus, it results in low material utility rate throughout the whole process, e.g. less than one third of the material, failing to satisfy the needs for mass production and at low costs.
A manufacturing method using stamping is disclosed in Taiwan Patent No. 00535010. A stamp with protruded blocks is stained with dyeing materials and the stamp is pressed to define a micro-structure pattern on a transparent insulating substrate which is then baked. The procedures are repeated three times to produce patterns with RGB color blocks. Despite of the advantages of high material utility rate and low manufacturing cost, this method provides limited variation of patterns, making it difficult to change the arrangement of the arrays of pixels at liberty.
An ink injection method is taught in Taiwan Patent No. 00512242. The ink injection method allows direct control on the positioning of ink injecting head module for defining patterns. The procedures of the method are as follows: coating a layer of absorbing film on a glass substrate to secure the absorption of the ink droplets to the glass substrate; next, allowing the ink injecting head module to directly spray the RGB color ink droplets onto the glass substrate to define the patterns required. This ink injection method has solved the problem of low material utility rate encountered in the conventional spin coating and photolithography, allowing higher extent of pattern variation than the stamping method.
However, since the ink injection method basically forms a line or surface pattern by a numerous dots, each droplet must be injected with extremely high accuracy into a block of a few microns or even smaller dimension. Besides, the traveling paths of droplets are susceptible to air flow disturbance, and it is likely that the ink droplets are injected accidentally to adjacent blocks and results in contamination. Therefore, a high precision machine is required. Also, the moving rate of the ink injecting head module is limited to secure precise injection. This can be what holds up the application of the method in industry. Because each of ink injecting heads is allowed to jet only one droplet at one time, the production efficiency is very low. In order to solve this problem, the numbers of the ink injecting heads have to be increased (which inevitably increase the cost). Besides, when ink injections are taking place in parallel movements, all ink injecting heads have to be in good condition without any clogging or abnormal situation. When the ink injection method is applied in large sized display panels, an enlarged dimension of machine is used. It should be careful to maintain good machine mobility and coating uniformity. These problems are yet to be solved in the future when large dimension TV displays will become the major products.
Thus, it is desired to develop a coating method that is simple in operation, has good yield rate and is economical for application.